1. Field of the Invention
The present invention relates to a color separator and an imager for use to capture an image or video.
2. Description of the Related Art
Digital still cameras and digital camcorders for acquiring digital video data by capturing an image or video have become more and more popular recently. As methods for capturing a color image or video, a space division technique and a time division technique are known.
According to the space division technique, the given video is spatially decomposed into respective colors using RGB color filters and the intensities of the respective colors are detected in respective pixels, thereby obtaining digital image data. A lot of digital cameras and digital camcorders of today adopt this technique.
On the other hand, according to the time division technique, the given video is temporally decomposed into respective colors during a predetermined period (e.g., a frame period) and the intensities of respective colors are detected in the same pixel, thereby obtaining digital image data. Hereinafter, a conventional imager that adopts this time division technique will be described by reference to Japanese Patent Application Laid-Open Publication No. 9-172649. FIG. 6 schematically illustrates an overall configuration for the conventional imager. Natural light or any other type of light is incident on an object 1, reflected from the object 1, transmitted through a color separating plate 13, and then imaged by a lens system 7, thereby producing an image 9 on the photodetector 8 of a CCD or a CMOS device. To achieve required optical performance with the lens system 7, a number of lenses are usually arranged along the optical axis. In FIG. 6, however, only one lens is illustrated for the sake of simplicity. The color separating plate 13 is obtained by dividing a transparent disk into four areas with four lines 13a, 13b, 13a and 13d extending radially from its center O and by providing color filters for the respective areas 13R, 13G, 13G′ and 13B. Specifically, a filter transmitting a red ray only is provided for the area 13R, filters transmitting a green ray only are provided for the areas 13G and 13G′ and a filter transmitting only a blue ray is provided for the area 13B.
The color separating plate 13 is secured to a motor so as to rotate around the axis L that passes the center O as pointed by the arrow 15 in FIG. 6. Accordingly, the image 9 produced on the photodetector 8 changes its colors in the order of red, green, blue and green time-sequentially.
The photodetector 8 detects these color rays and sends out signals representing those colors to an external computing unit. The computing unit calculates those colors as a set of signals representing red, green and blue and also calculates the brightness as a signal representing green. In this manner, a color image is reproduced.
According to this time division technique, there is no need to provide color filters for respective pixels. In addition, since the three primary colors of R, G and B can be all detected in a single pixel, the pixel pitch can be narrowed and a video with high resolution can be obtained.
In the imager disclosed in Japanese Patent Application Laid-Open Publication No. 9-172649, however, the color separating plate 13 should rotate around the axis L, which is arranged outside of the optical path of the lens system 7 so as to avoid interference with the lens system 7. Thus, the overall size of the imager increases.
In addition, while the color separating plate 13 is rotating, there is a period in which the division line 13a, 13b, 13a or 13d splits the cross section BS of the optical path 2 of the lens system 7 into two on the color separating plate 13. During such a period, the image 9 produced on the photodetector 8 is presented in two colors and the boundary between the colors shifts, too. The photodetector 8 cannot acquire normal data during that period and needs to perform its detection while none of the division lines 13a, 13b, 13c and 13d is splitting the cross section BS, thus decreasing the optical efficiency.
The optical efficiency is proportional to R/r, where R is the distance from the center O of rotation to the center S of the cross section BS and r is the radius of the cross-sectional area BS. However, when a lens system is adopted, the magnitude of r is determined automatically. That is why the optical efficiency is proportional to the magnitude of R. Consequently, to increase the optical efficiency, the color separating plate 13 needs to have an increased size, thus making it difficult to reduce the size of the imager.